The Study of Heredity Chapter 2

Basics of Heredity Blending Theory Gregor Mendel Early attempt at explaining family resemblance However, traits would not persist from generation to generation Gregor Mendel Augustinian friar from what is now the Czech Republic Founder of modern genetics Pea plant studies 1856-1863 demonstrated the basic concept of inheritance

Work of Gregor Mendel Crossed true-breeding plants to create hybrids Hybrids – Offspring of parents that differ from each other with regard to certain traits or certain aspects of genetic makeup Allowed the hybrid plants to self-pollinate ¾ of one trait, ¼ of the other Dominant and recessive traits Dominant traits are governed by an allele that can be expressed in the presence of another Recessive traits are not expressed in heterozygotes The principles of genetics were outlined by Mendel, but he was unaware of the biological nature of the genetic material

Two Principles of Mendelian Genetics Principle of Segregation Counters the idea of blending inheritance For every trait, an individual has two hereditary factors, one inherited from each parent Together, these cells determine the appearance of the trait in the organism Principle of Independent Assortment Inheritance patterns of differing traits are independent of one another The distribution and selection of traits is random

Segregation

Alleles and Genes Genes Alleles A stretch of DNA or RNA that determines a trait Alleles A number of alternative forms of the same gene Ex: Gene – Eye Color; Allele – Brown, Blue, Green, etc. Can occur at the same locus on a pair of chromosomes and influence the same trait An allele is a fixed spot on a chromosome Diploids – organisms with two alleles at each locus One allele from each parent (humans)

Genotype and Phenotype The genetic makeup of an individual Can refer to an organism’s entire genetic makeup or to the alleles at a particular locus Phenotype The observable or detectable physical characteristics of an organism The detectable expressions of genotypes, frequently influenced by environment

Simple Mendelian Inheritance Heterozygous Having different alleles at the same locus on members of a pair of chromosomes Homozygous Having the same allele at the same locus on both members of a pair of chromosomes Recessive homozygotes – two recessive alleles Dominant homozygotes - two dominant alleles There are a number of human characteristics that are inherited in a simple Mendelian manner Human earlobe can be either free-hanging (dominant) or attached (recessive)

Chromosomes Genetic structure composed of a DNA molecule and associated proteins During cell division, chromosomes consist of two strands of DNA joined at the centromere Act as the site of assembly Since the DNA molecules have replicated, one strand of a chromosome is an exact copy of the other

Human Chromosomes Both sexes have 22 pairs of non-sex chromosomes, referred to as autosomes, and a 23rd pair of sex chromosomes The normal female possesses two homologous sex chromosomes (XX) while the normal male has only one X chromosome which pairs with a different type, the Y chromosome

Cell Division Mitosis Meiosis Steps: Cell division in somatic cells Occurs during growth and repair/replacement of tissues The result is two identical daughter cells that are genetically identical to the original cell Steps: The 46 chromosomes line up in the center of the cell The chromosomes are pulled apart at the centromere The strands separate and move to opposite ends of the dividing cell The cell membrane pinches in and two new cells exist Meiosis Cell division in specialized sex cells in ovaries and testes Involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes These cells can develop into gametes Recombination - the exchange of genetic material between partner chromosomes during meiosis

Heredity and DNA Understanding the structure of the DNA molecule is also crucial to our comprehension of heredity Cellular function and an organism’s inheritance depends on the structure and function of DNA DNA is composed of two chains of nucleotides, comprising a double strand or double helix A nucleotide consists of a sugar, a phosphate, and one of four nitrogenous bases (bases)

The DNA Molecule James Watson (left) and Francis Crick in 1953 with their model of the structure of the DNA molecule.

DNA Structure Nucleotides form long chains The two chains are held together by bonds formed on their bases with their complement on the other chain Adenine(A) is the complement of Thymine(T) Guanine(G) is the complement of Cytosine(C) DNA bases form base pairs in a precise manner. Adenine can bond only to thymine, etc.

DNA Replication The DNA molecule is able to make copies of itself Steps: Enzymes (specialized proteins) break the bonds between the DNA molecule Two nucleotide chains serve as templates for the formation of a new strand of nucleotides Unattached nucleotides pair with the appropriate complementary nucleotide

RNA and DNA RNA differs from DNA in three important ways: It is usually single-stranded (This is true of the forms we discuss, but it is not true for all) It contains a different type of sugar, ribose. It contains the base uracil as a substitute for the DNA base thymine (Uracil is attracted to adenine, just as thymine is)

Protein Synthesis Process by which cells build proteins Proteins Long chains of basic units known as amino acids The blueprint for a specific protein is located in the DNA Messenger RNA (mRNA) carries the genetic message A form of RNA assembled on a sequence of DNA bases It carries the DNA code to the ribosome during protein synthesis Transfer RNA (tRNA) binds to one specific amino acid Transcription The process of coding a genetic message for proteins A portion of the DNA unwinds and serves as a template for the formation of a mRNA strand